1. Field of Invention
This invention generally relates to electrostatic printing apparatus and methods of electrostatic printing. In particular, the invention relates to methods and apparatus for enhancing the custom color capabilities of an electrostatic printing apparatus.
2. Description of Related Art
Electrostatographic printing is well known and commonly used for copying or printing documents on a paper substrate. Electrostatographic printing is performed by forming a substantially uniform charge on a photoconductive member and exposing the photoconductive member to a pattern of light. Exposing the photoconductive member to the pattern of light generates a corresponding electrostatic latent image on the photoconductive member. Toner particles are then deposited onto the photoconductive member so that the toner particles are selectively deposited in either charged or discharged areas on the photoconductive member. The developed toner image is then typically transferred to a substrate and fixed to the substrate by heat and/or pressure. The photoreceptor is then cleaned of any residual toner or electric charge in preparation for another charge/electrostatic latent image generating/development process.
Electrostatographic printing systems typically develop an electrostatic latent image using solid toner particles either in powder form or suspended in a liquid carrier. In liquid developing systems, the liquid developer typically has about two percent by weight toner material distributed in the liquid carrier. An electrostatic latent image is developed by applying the liquid developer to the photoconductive member, whereby the toner particles are selectively attracted to the surface of the photoconductive member in accordance with an electrostatic latent image.
Conventional liquid printing systems, such as liquid immersion development (LID) systems, can generate custom colors by combining two or more primary color toners before depositing the toners and then using the mixed toner to develop an electrostatic latent image. However, due to the differences in physical and chemical properties of the toners of different colors and other factors, a sophisticated feedback scheme must be used to obtain accurate color reproduction and color stability. For example, the differential mobility of the mixed toners often results in different consumption rates of different toner during development, requiring complex color control techniques to maintain a desired composition, e.g. color, of the toner and the color and density of the toner image created.
The on-demand custom color capability of electrostatographic printing systems may vary significantly due to numerous conditions affecting image development, among various factors, including but certainly not limited to the methods and apparatus used to mix the primary colors to achieve the desired custom color and the process controls implemented on the color mixing and development subsystems to maintain the color accuracy and stability. In general, a number of primary color developers are mixed in a reservoir with certain proportions according to the customer selection and the consumption rate of the primary colors, and then the developer mixture is applied to the latent image for development. Examplary patents which may describe certain general aspects for achieving customer selectable colors, as well as specific apparatus therefor, may be U.S. Pat. No. 5,781,828 to Caruthers et al., as well as other patents cited therein.
The relevant portions of the foregoing patent may be briefly summarized as follows:
U.S. Pat. No. 5,781,828 discloses a system and method for color mixing management in an electrostatographic printing system, wherein a developing material reservoir containing an operative solution of colored developing material including a mixture of selected color components is continuously replenished with selected differently colored developing material concentrates in a predetermined ratio so as to be capable of producing a customer selectable color image area on an output substrate. The system may also be used to mix a customer selectable color in situ, either from stored proportions known to compensate for developability differences or from approximate amounts of primary color components initially deposited and mixed in the developing material reservoir with the resultant operative developing material mixture continually developed and replenished with a predetermined ratio of color components until the developing material mixture reaches a steady state color.